1. Field of the Invention
The present invention relates to the field of communication systems, and more particularly to communication systems for transmitting and receiving useful information in the presence of varying signal fading and drop outs.
2. Prior Art
Apparatus and techniques for radio communication between two points are well known in the prior art. While such communication is most commonly by way of modulating a voice signal on a carrier, techniques are also well known for modulating digital information in serial form on an appropriate carrier for radio transmission. The most common form of such transmission is a fixed point to point transmission, with the locations of the transmitter and receiver and the transmitter power being selected so as to provide an adequate signal strength at the receiver under substantially all necessary atmospheric conditions.
In certain situations the transmitter or receiver may be mobile (submarine-space-marine-land), such as by way of example, in police cars, public transportation vehicles, emergency vehicles, etc. Accordingly, signal reflections from buildings and the like and atmospheric conditions may temporarily cause the original signal and a reflected signal to arrive at the receiver with substantially the sme amplitude but with opposite phasing, so as to result in serious signal fading and drop out.
There are two approaches which have been commonly used, either alone or together, to minimize the effects of these greatly varying signals. The first is to modulate the information to be transmitted such as the voice signal onto two carriers of differing frequencies and to transmit and receive both signals. Since the two signals have different wave lengths because of their different frequencies, it is unlikely that cancellation of both signals will occur simultaneously, so that the strongest signal or some combination of the two signals will provide a much more reliable signal than either signal individually. The second approach is to provide a plurality of receiving stations physically distributed around the expected area of travel of the transmitting station, again using either the strongest signal received or some combination of signals from all receivers. This space diversity, of course, also enhances the ability to derive a useful signal, as signal drop out at physically diverse locations at the same time is increasingly unlikely.
When using a multiplicity of space diversity, polarization diversity, time diversity and frequency diversity, the likelihood is very high that at least one spatial-temporal signal is of reasonably high quality for one time increment or message block. The problem however is to select each acceptable short message block from the multitude of signals being received and then assemble the short acceptable blocks into the complete message. In the prior art in the case of voice communication, various techniques have been used to attempt to identify the best signal. Rapid and accurate identification of the best signal block is most difficult. In many cases the apparent best signal is manually selected by switching between channels to find one of the demodulated signals which is sufficiently intelligible for the intended communication purposes. However this is rather tedious, particularly under rapidly varying conditions, so that frequently a less than best signal is used until the extent of fade-out is intolerable.
Certain automatic techniques have also been used for selecting the apparent "best" channel which represent a substantial improvement over the manually switched systems. These prior art systems generally depend upon some signal amplitude responsive measurement to either select the best apparent signal or to provide automatic gain control for each of the signals so that the combined signal emphasizes the better signals. However, since noise cannot be eliminated, such systems are typically responsive either to noise or signal plus noise rather than the quality or information content of the signal alone. Examples of systems directed to the problems of this type of communication are the systems disclosed in U.S. Pat. Nos. 2,034,738 (Beverage); 2,642,524 (Bayliss); 2,899,548 (Boughtwood et al); 3,035,169 (Griffith); 3,403,341 (Munch); 3,495,175 (Munch); 3,618,067 (De Vale et al.); 3,718,889 (Rollins).